Recent advances in the application of solution NMR spectroscopy to multi-span integral membrane proteins

Integral membrane proteins (IMPs), which can only be released from the membrane by disruption of the membrane, perform a host of vital cellular functions as receptors, transporters, channels, electrical and photo-transducers. The aggregate molecular weight of an IMP-model membrane complex offers some insight into the challenge that a solution NMR structural effort will present under the most favorable circumstances. Cell-free systems have emerged as a promising alternative for preparing large quantities of isotopically labeled membrane proteins. Higher complexity eukaryotic hosts have also been used for producing protein samples for NMR characterization. These systems include baculovirus-infected insect cells and transfected mammalian cells. Successful NMR studies of IMPs have been carried out in organic solvent mixtures. A report from MacKenzie and co-workers has demonstrated that the addition of very modest amounts of phospholipids to micelles can result in the enhancements of NMR spectral quality for some integral membrane proteins.